1. Field of the Invention
This invention relates to a laser beam generator and, more particularly, to a laser beam generator for generating a laser beam having a wavelength translated by a non-linear optical crystal element.
2. Background of the Invention
It has hitherto been proposed to achieve efficient wavelength conversion by taking advantage of the high power density in a resonator. For example, researches have been conducted for second harmonic generation (SHG) by an external resonator type device and SHG by a device having a non-linear optical element provided within the resonator.
As an example of a generator of the type of SHG within the laser resonator is a generator in which a laser medium and the non-linear optical crystal element are arranged between a pair of reflecting mirrors as components of the resonator. With the type of the laser beam generator, the second harmonic laser beam is phase-matched with respect to the fundamental laser beam within the non-linear optical crystal element within the resonator for efficiently taking out a second harmonic laser beam.
For achieving phase matching, it is necessary to set up type I or type II phase matching conditions between the fundamental laser beam and the second harmonic laser beam. The type I phase matching is based on a principle of creating a phenomenon of producing a photon of a doubled frequency from two photons polarized in the same direction by taking advantage of an ordinary light beam of the fundamental laser beam. With the type II phase matching, on the other hand, two fundamental waves of proper polarization perpendicular to each other are caused to fall on a non-linear optical crystal element for setting up phase matching conditions for each of these beams. With the type II phase matching, the fundamental wavelength laser beam is split by in the non-linear optical crystal element into an ordinary light beam and an extraordinary light beam to effect phase matching with respect to the extraordinary light beam of the second harmonic laser beam.
However, if desired to generate the second harmonic laser beam using the type II phase matching conditions, proper polarization of the fundamental laser beam undergoes phase changes each time the fundamental laser beam is repeatedly passed through the non-linear optical element, as a result of which the second harmonic laser beam tends to be generated only unstably if the type II phase matching conditions are employed.
That is, if proper oscillations perpendicular to each other, that is p-wave component and s-wave component, are deviated progressively in phase each time the fundamental laser beam generated by resonation in the laser medium is passed through the non-linear optical crystal element, it becomes impossible to realize a steady-state condition in each part of the resonator in which the laser light beams efficiently strengthen one another, such that a state of strong resonation, that is a strong standing wave, cannot be produced within the resonator. The result is that the translation efficiency from the fundamental laser beam into the second harmonic laser beam is deteriorated and the noise tends to be produced in the second harmonic laser beam.
The present Assignee has already proposed in JP Patent KOKAI Publication No.1-220879 (1989) a laser beam generator in which a birefringent element, such as a quarter wave plate, is inserted in a resonation light path of a fundamental laser beam in a laser source adapted for generating a second harmonic laser beam by a non-crystal optical element for stabilizing the second harmonic laser light radiated as an output laser beam.
FIG. 1 shows a typical laser beam generator as disclosed in JP Patent KOKAI Publication No.1220879. The laser beam generator shown therein has a resonator 201 comprised of a reflecting surface 203, such as a dichroic mirror formed on the incident surface of a rod-shaped laser medium 202, such as Nd:YAG laser medium, and another reflecting surface, such as a dichroic mirror, formed on the inner surface of an output concave mirror 104. The Nd:YAG laser medium 202, a non-linear optical crystal element 206, formed of KTP(KTiOPO.sub.4), and a birefringent element 207, are arrayed within the resonator 201. The birefringent plate 207 is formed by a quartz plate designed as a quarter wave plate, for producing a phase difference of one-quarter cycle of the fundamental laser beam generated in the laser medium 202 within the resonator 201. The laser medium 202 generates the fundamental laser beam by a pumping light from the pumping semiconductor laser 211 falling on an incident surface 203 of the laser medium 202 via collimator lens 212 and object lens 213. The fundamental laser light is transmitted through non-linear optical crystal element 206 and birefringent plate 207 so as to be reflected by the reflecting surface of the concave mirror 204. The fundamental laser beam is again transmitted through birefringent element 207, non-linear optical crystal element 206 and laser medium 202 in this order so as to be reflected by the reflecting surface 203. Thus the fundamental laser light travels back and forth between the incident reflective surface 203 of the laser medium 202 and the inner reflecting surface of the output concave mirror 204 within the resonator 201 by way of performing a resonant oscillation.
The birefringent element 207, such as the quarter wave plate, has its optical axis set so that the direction of the refractive index for extraordinary light n.sub.e(7) is inclined a predetermined azimuth angle, such as azimuth angle .theta.=45.degree., with respect to the direction of the refractive index of the non-linear optical element 206 for extraordinary light n.sub.e(6), within the plane perpendicular to the oscillation of the amplitude of the light beam, as shown in FIG. 2.
In the above-described laser beam generator, the second harmonic laser beam is generated as the fundamental laser beam is passed through the resonant light path via the non-linear optical crystal element 206. The second harmonic laser beam is transmitted through the concave mirror 204 so as to be radiated as output laser beam.
It is noted that the light beams making up the fundamental laser beam are transmitted trough the birefringent element 207, adjusted to an azimuth angle .theta.=45.degree. with respect to the non-linear optical crystal element 206, for stabilizing the laser beam power at each part of the resonator to a predetermined level. That is, as the fundamental laser beam generated the laser medium 202 is passed through the non-linear optical crystal element 206 by way of resonant oscillation to produce a type II second harmonic laser beam, coupling by sum frequency generation between the two polarization modes of the fundamental laser beam perpendicular to each other is inhibited, as a result of which the oscillation of the second harmonic laser beam may be stabilized. If the two proper polarization modes of the p-wave component and the s-wave component are of equal intensity, the spatial phase difference between the two proper polarization modes becomes equal to 90.degree., as shown in FIG. 3(A). The result is that the two polarization modes are simultaneously set into oscillation so that the standing wave in the resonator becomes uniform in light intensity, as shown in FIG. 3(B). In this manner, the spatial hole burning effect, which indicates axial spatial non-uniformity of the oscillation gain, may be inhibited to produce stable double longitudinal mode oscillation.
Meanwhile, the quarter wave plate 107, as the birefringent plate, has its both sides coated with anti-reflection (AR) coating to permit 100% transmission of the fundamental laser beam having a wavelength of 1064 nm. In other words, the birefringent element 107 has its both sides coated with AR coating to permit 0% reflection of the fundamental laser beam having a wavelength of 1064 nm. However, in effect, the AR coating of the quarter wave plate 107 is subject to residual reflection such that 0% reflectance cannot be achieved and residual reflection R on the order of 0.1% is incurred. By this residual reflection R, multiple reflection is incurred within the quarter wave plate. With the wavelength .lambda., the thickness of the quarter wave plate D and with the refractive index of the quarter wave plate n, the reflectance R.sub.m of the multiple reflection may be expressed by ##EQU1## where .DELTA.=4.pi.nD/.lambda..
Therefore, if fluctuations in the effective thickness of the quarter wave plate on the order of the quarter wave plate thickness are changed from 0 to .lambda./4 due to thermal expansion or manufacture tolerances, the reflectance R.sub.m is changed from 0 to about 4 R. Since the quarter wave plate has a difference in thickness of one-quarter of a wavelength with respect to the incident polarized light, a difference in the loss within the resonator of about 4 R at a maximum is produced between the two polarization modes.
If there is such difference in the loss within the resonator between the two polarization modes, there is incurred a difference in intensity between the two modes of proper polarization, that is the p-wave component and the s-wave component, as shown in FIG. 4(A), so that the standing wave within the resonator becomes non-uniform, as shown in FIG. 4(B), to incur the above-mentioned spatial hole burning effects. If double or more longitudinal modes are oscillated, there may be occasions wherein the oscillation becomes non-uniform due to coupling by sum frequency generation between the longitudinal modes of the same polarization, by reason of the nonuniform intensity of the standing wave.
Meanwhile, in the above-described resonator of the standing wave type in which the light beam is caused to travel back and forth repeatedly between two mirrors, since the fundamental laser beam falls on the non-linear optical crystal element as it travels back and forth repeatedly between the mirrors, the second harmonic laser beams are generated in two directions with respect to the non-linear optical crystal element. For example, in the basic resonator shown in FIG. 5 in which a non-linear optical crystal element 222 and a laser medium 223 are provided on a light path in the resonator made up of an optical element 221 having a reflecting surface 221R for transmitting 100% of the second harmonic laser beam having the wavelength of 532 nm and reflecting 100% of the fundamental laser beam having the wavelength of 1064 nm and an optical element 224 having a reflecting surface 224R for transmitting 100% of the second harmonic laser beam having the wavelength of 532 nm and reflecting 100% of the fundamental laser beam having the wavelength of 1064 nm, the fundamental laser beam generated in the laser medium 223 is incident on the non-linear optical crystal element 222 as it caused to travel back and forth repeatedly between the reflecting surfaces 221R and 224R, so that the second harmonic laser beam generated in the non-linear optical crystal element 222 is radiated in two directions, that is towards the reflecting surface 221R and towards the reflecting surface 224R.
However, it is difficult to realize a mirror coating reflecting 100% of the fundamental laser beam and transmitting 100% of the second harmonics, such that several to tens of percents of reflection of the second harmonic laser beam is necessarily incurred, as shown in FIG. 6 which illustrates characteristics of the mirror coating applied to a quartz surface. For example, transmittance to the second harmonic laser beam having a wavelength .lambda. of 532 nm is about 97%, such that about 3% of the beam is reflected. On the other hand, transmittance to the fundamental laser beam having a wavelength .lambda. of 1064 nm is about 99.91%.
If, with a reflecting surface on which a coating is applied to permit approximately 100% transmission of the second harmonic laser light, 1% of the light is reflected due to manufacture tolerances, the reflected light in the amount of 1% is superimposed on the other second harmonic laser beam to produce interference.
The phase of the reflected light is usually constant at all times if the fundamental laser beam and the second harmonic laser beam have the same speed of propagation and both of these laser light beams are not subject to dispersion. However, since dispersion is incurred by air, non-linear optical crystal element and by the laser medium, the reflected light is changed in phase because of changes in temperature. When the reflected light beam is changed in phase in this manner, the forward light intensity I given by ##EQU2## is incurred, where w and R indicate the phase and the reflectance of the reflected light, respectively.
The following Table 1 shows values of amplitudes of intensity fluctuations (2.sqroot.R) when the phase of reflected light is changed from 0 to .pi. for several values of reflectance R of the reflected light, as calculated by the formula (2), and the backward intensity (=1-R).
TABLE 1 ______________________________________ amplitude 2.sqroot.R of reflectance R of intensity backward intensity reflected light fluctuations (= 1 - R) ______________________________________ 10% 67% 90% 1% 20% 99% 0.1% 6.7% 99.9% 0.01% 2% 99.99% ______________________________________
It is seen from Table 1 that fluctuations in intensity amounting to .+-.20% are incurred even with the reflectance R of the reflected light of 1%. That is, with the backward intensity (1-R)=99% for the reflectance R of 1%, its backward output obviously differs from the forward effective output accompanied by fluctuations in intensity amounting to .+-.20%. Consequently, if it is attempted to control the intensity of the light of the forward effective output in the laser beam generator shown in FIG. 5 based on the detected value of the intensity of the backward output, it is difficult to maintain a constant output because the forward and backward second harmonic laser light beam intensities are not coincident with each other by interference effects.
On the other hand, it is also seen from Table 1 that, if the intensity fluctuations should be maintained within a range of .+-.2%, it is necessary to keep the reflectance R of the reflected light within 0.01%. That is, the changes in intensities cannot be maintained within 2% with a mirror having the mirror coating having the characteristics as shown in FIG. 6.
Although a part of the forward output may be split by a beam splitter and detected by a photodetector, the number of optical components is correspondingly increased, while an output efficiency is lowered.